1. Field of the Invention
The present invention relates to a method and apparatus for monitoring cationic conductivity, which are used to sense the concentration or inflow of impurities in circulating water of a circulating water system. More particularly, the present invention relates to an improved method which remarkably improves the measurement sensitivity for cationic conductivity by utilizing a mixed-type column where cation exchange resin and anion exchange resin are present together, and also permits an apparatus to carry out the same.
2. Background of the Related Art
Generally, impurities (salts) in water consist of cations including sodium, calcium and magnesium, and anions including sulfate ion and carbonate ion. An increase in concentration of impurities in system circulation water results in acceleration of corrosion or production of scales, thereby reducing equipment lifespan and lowering heat efficiency. For this reason, concentration of the impurities in water needs to be maintained at low level. In most industrial fields, industrial water containing large amounts of the impurities is used as cooling water for a heat exchanger. Thus, if fine holes caused by corrosion, etc. are formed on a cooling tube, the impurities are introduced or concentrated in the system so that concentration of the impurities is liable to increase. For this reason, the construction of a system for regularly monitoring the impurities in water is very critical.
A conventional method and apparatus for measuring cationic conductivity, which are most generally used for monitoring impurities in water, will now be described with reference to FIG. 1. In such a method for measuring the cationic conductivity, sample water 1 is passed into a column filled with a cation exchange resin via a valve 2 and a flow meter 3, so that cations (e.g., Na+) constituting the impurities in the sample water 1 is converted into hydrogen ions (H+), and anions of the impurities remain intact and are finally converted into hydrochloric acid (HCl), thereby highly increasing conductivity of the impurities. The increased conductivity is measured by a measuring unit comprising an electrode 5 and an indicator 5, whereby impurities are detected.
Table 1 below shows equivalent conductivity of anions and cations which are generally present in water. For example, where water containing salts (NaCl) is introduced and passed through a cation exchange column, sodium ions (equivalent conductivity: 50.1) as cations are substituted with hydrogen ions (equivalent conductivity: 49.8), thereby highly increasing conductivity of the impurities. Namely, according to reaction represented by the following reaction equation, the equivalent conductivity before passage through the cation exchange column is 126.45 (50.1+76.35) but the equivalent conductivity after passage through the cation exchange column is 426.15 (349.8+76.35).
Na++Cl−+R—H (hydrogen-type cation exchange resin)→H++Cl−+R—Na Thus, as conductivity the impurities is highly increased, a content of impurities can be easily detected by measuring an increase in conductivity.
TABLE 1Equivalent Conductivities of Various Ions (mho · cm2 · eq2) at 25° C.EquivalentEquivalentCationsconductivityAnionsconductivityNa+50.1Cl−76.35Ca2+59.5SO42−80.02H+349.8CO32−69.3NH4+73.55HCO3−44.5
Generally, seawater contains anions in the order of chlorine ions, sulfate ions and carbonate ions in amount, and freshwater contains anions in the order of carbonate ions, chlorine ions and sulfate ions in amount. Where sample water is passed through the cation exchange column, cations in impurities are substituted with hydrogen ions, so that chlorine ions, sulfate ions and carbonate ions (or bicarbonate ions) are converted into hydrochloric acid, sulfuric acid and carbonic acid, respectively.
As indicated in Table 2 below, hydrochloric acid and sulfuric acid, that are strong acids, are nearly completely dissociated due to their high dissociation degree and thus contribute to increasing conductivity of the impurities. However, since carbonic acid, that is a weak acid, very partially contributes to increasing conductivity of the impurities, it shows low conductivity even when it is present in water at high concentration.
TABLE 2Dissociation Constant of Various AcidsAcidsChemical formulaDissociation constantHydrochloric acidHCl1.26 × 105Sulfuric acidH2SO4     103Carbonic acidH2CO3 4.3 × 10−7Phosphoric acidH3PO4 7.5 × 10−3Fluoric acidHF 2.7 × 10−2
Seawater contains the impurities at a relatively high concentration so that the impurities can be easily detected even when they are introduced at a very small amount. However, freshwater contains the impurities at a relatively low amount. Thus, where cooling water is introduced into a system in an industrial field using freshwater as cooling water (e.g., cogeneration power plant), it is disadvantageous in that monitoring of the inflow of cooling water is difficult due to its low detection sensitivity even when the measurement of conductivity is carried out using the cation exchange column.
Furthermore, in a method for monitoring the inflow of air or organic substances into a system according to the present invention, conductivity of water effluent from the cation exchange column is measured for detection of carbon dioxide (about 350 ppm) present in air or for detection of carbon dioxide produced by decomposition of organic substances. Then, the effluent water is heated to the boiling point to remove carbonate ions, after which conductivity of water is measured again. From a difference between the two measured conductivities, concentration of carbonate ions is measured. However, this method has problems in that it is complicated and real-time measurement is made impossible.